1. Field of the Invention
This invention relates to deck plate covers in general and to lockable manhole covers in particular.
2. Description of the Prior Art
Unauthorized removal of manhole covers has been a problem for over a century. For example, manhole covers have been stolen for their value as scrap iron and have also been frequently removed by unauthorized persons who can then access the underground passageways that often contain valuable installations, such as electrical and communications systems, water lines, natural gas lines and the like. A serious problem which has developed recently is the unauthorized removal of manhole covers for the purpose of surreptitiously dumping toxic or hazardous wastes into underground sewer systems. Unlockable manhole covers, while being heavy and awkward, are not particularly difficult to remove as they simply rest on a ledge or flange protruding from a frame or base surrounding the manhole opening.
In response to these problems, several locking manhole covers have been developed over the past century. Many different designs exist, but the most popular designs use at least two locking rods extending radially outward from the underside of the covers to engage the flanges in the support frames, thus locking the covers in place. The rods are extended and retracted by some means, usually a center crank that converts a rotational movement of the crank to a linear movement, which extends and retracts the rods. Another apparatus uses a dog lock device diametrically situated on and extending through the manhole cover to lock it in position.
While these systems succeeded in locking the cover to the manhole, they were usually not very tamper-resistant or required as many as three separate tools to open. Usually the degree to which a given design was tamper-resistant was directly proportional to the complexity, and therefore cost, of the design.